Research Article

Complete chloroplast genome sequence of cultivated Morus L. species

Published: October 17, 2016
Genet. Mol. Res. 15(4): gmr15048906 DOI: https://doi.org/10.4238/gmr15048906
Cite this Article:
(2016). Complete chloroplast genome sequence of cultivated Morus L. species. Genet. Mol. Res. 15(4): gmr15048906. https://doi.org/10.4238/gmr15048906
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Abstract

The complete chloroplast genome (cpDNA) sequences of two cultivated species of Morus L. (Morus atropurpurea and Morus multicaulis) are reported and reconstructed in this study, and were compared with that of wild Morus mongolica. In M. atropurpurea, the circular genome is 159,113 bp in size and comprises two identical inverted repeat (IR) regions of 25,707 bp each, separated by a large single-copy (LSC) region of 87,824 bp and a small single-copy (SSC) region of 19,875 bp. The cpDNA sequence of M. multicaulis is longer than that of M. atropurpurea (159,154 bp), and consists of two IRs (25,678 bp), a LSC region (87,763 bp), and a SSC region (20,035 bp). Each cpDNA contains 112 unique genes including 78 protein-coding genes, 30 transfer RNA genes, and 4 ribosomal RNA genes, with a GC content of 36.2%. There were 83 simple sequence repeats (SSRs) with mononucleotides being the most common (60) and di-, tri-, tetra-, and hexanucleotides appearing less frequently in M. atropurpurea. M. multicaulis contains 81 SSRs containing 63 mononucleotide repeats. The genes and SSRs identified in this study may enhance understanding of cpDNA evolution at both intra- and interspecific levels. MEGA 6.0 was used to construct a phylogenetic tree of 27 species, which revealed that M. atropurpurea and M. multicaulis are more related to their congeners than to others. The cpDNA of M. atropurpurea and M. multicaulis and its structural analysis are important for the chloroplast genome project, development of molecular markers for Morus species, and breeding of varieties.

INTRODUCTION

The chloroplast (cp) is the photosynthetic organelle representing one of the most important organelles in green plants and algae. Its genome has proven to be useful for plant phylogenetics, species identification, population genetics, and genetic engineering (Nock et al., 2014). In angiosperms, the chloroplast genome (cpDNA) is typically composed of a pair of inverted repeat regions (IRa and IRb), which are separated by a small single-copy (SSC) region and a large single-copy (LSC) region (Jansen and Palmer, 1987; Wu et al., 2009).

The length of cpDNA ranges from 120 to 160 kb, owing to the loss and gain of introns (Delannoy et al., 2011), the expansion of the IR region (Dong et al., 2013; Zhang et al., 2013), and major structural rearrangements (Walker et al., 2014), which contain 110 to 130 genes (Huang et al., 2013). Chloroplasts are a valuable tool for use in phylogenetic studies because of their genes, which lack recombination and conversation (Ravi et al., 2008). To date, more than 1000 complete cpDNA sequences have been submitted to GenBank; however, the cpDNA sequence of Moraceae is incomplete. The cpDNA of the cultivated species Morus atropurpurea and M. multicaulis are described in detail in this study.

Morus L. is an economically significant crop belonging to the Moraceae family, which was once classified in the subclass Hamamelidae (Order: Urticales) (http://plants.usda.gov/), but has now been reclassified in the order Rosales in Fabidae (also known as Rosid I) according to some of its nuclear genes or cpDNA sequences (Zhang et al., 2011; Su et al., 2014). There are 68 species of mulberry, which are found mostly in Asia, mainly China and Japan, and continental America, and include cultivated (M. atropurpurea and M. multicaulis) and wild (M. mongolica and M. notabilis) species. This family is poorly represented in Africa and Europe and is virtually absent from Australia. Their leaves provide the sole source of food for the silkworm and their fruits are rich in nutrients and are beneficial to human health. Although there have been a few phylogenetic studies involving mulberry, these were restricted to only a few genes. A complete repertoire of genes would thus help us to establish the position of mulberry in the tree of life (Ravi et al., 2006). M. atropurpurea and M. multicaulis are native to China and are cultured in Shaanxi Province. In this study, the cpDNA sequences of M. atropurpurea and M. multicaulis were investigated, and a comparative analysis was performed between cultivated Morus and M. mongolica. The genome structure, gene order, repeat sequences, and phylogenetics were analyzed.

MATERIAL AND METHODS

Plant material, sequences, assembly, and annotation

M. atropurpurea and M. multicaulis plants were collected from the mulberry field of Northwest A&F University. The DNeasy plant Minikit (Qiagen, Seoul, South Korea) was used to isolate total genomic DNA from 10 g fresh leaves and a UV-visible spectrophotometer was used to determine DNA concentration. High-quality DNA was sequenced using the Illumina Hiseq 2500 platform (Illumina Inc., San Diego, CA, USA).

The complete cpDNA sequence was assembled with MITOBIM V1.7 (Hahn et al., 2013) using default settings, with its congener M. mongolica (GenBank accession No. KM491711) as the reference sequence. Sequences were annotated in GENEIOUS R8 (Biomatters Ltd., Auckland, New Zealand) by aligning with that of M. mongolica. OGDraw was used to draw the circular gene map and ambiguous gaps or nucleotides were corrected manually (Lohse et al., 2007). Complete chloroplast genomes were submitted to GenBank under the following accession No. M. atropurpurea, KU355276 and M. multicaulis, KU355297.

Comparative analysis of Rosales chloroplast genomes

The mVISTA online software in shuffle-LAGAN mode (Frazer et al., 2004) was applied to compare the complete chloroplast genomes of cultivated Morus species with four representatives of Rosales: Prunus persica (Rosaceae: NC-014697), Pyrus pyrifolia (NC-015996), Fragaria vesca subsp vesca (NC-015206), and M. mongolica (Moraceae), with the basal species Nicotiana tabacum L. (Solanaceae; Solanales; Z00044) used as the reference in the comparative analysis.

Simple sequence repeats (SSRs) were identified using the online software Wabsat and Gramene Ssrtool (in the chloroplast genome of M. atropurpurea and M. multicaulis). A total of 10, 5, 4, 3, 3, and 3 SSRs were identified for mono-, di-, tri-, tetra-, penta-, and hexanucleotides, respectively.

Phylogenetic analysis

The MEGA 6.0 software was used to determine the phylogenetic relationships between Morus species by the maximum likelihood (ML) and neighbor-joining (NJ) methods (Tamura et al., 2013). Data on the cpDNA of Morus species are available, including those from M. indica (NC-008359), M. mongolica (KM491711), M. notabilis (KP939360), M. multicaulis (KU355297), and M. atropurpurea (KU355276), and N. tabacum, which was used as an outgroup. The likelihood bootstrap analysis of each branch was calculated with 1000 replications.

RESULTS

Genome content and organization

The cpDNA sequence of M. multicaulis was determined (Figure 1) and found to be 159,154-bp long, which is longer than that of the other congeners (Table 1). It comprises a circular double-stranded DNA structure composed of two identical IR regions (25,678 bp), a LSC region (87,763 bp), and a SSC region (20,035 bp). The M. atropurpurea cpDNA was found to be 159,113 bp long (Figure 2) and was composed of a typical quadripartite structure containing a pair of IR regions of 25,707 bp each, which were separated by a SSC region (19,875 bp) and a LSC region (87,824 bp) (Table 1).

Gene map of the chloroplast genome of Morus multicaulis.

Comparison of chloroplast genomes among five species of Morus L.

Characteristics M. indica M. mongolica M. notabilis M. atropurpurea M. multicaulis
Size (bp) 158,484 158,459 158,680 159,113 159,154
LSC length/percent/CG content 87,386/55.14/34.1 87,367/55.14/34.0 87,470/55.12/34.1 87,761/55.18/33.9 87,763/55.15/33.9
SSC length/percent/CG content 19,742/12.46/29.4 19,736/12.45/29.3 19,776/12.46/29.3 19,875/12.50/29.3 20,035/12.59/29.3
IR length/percent/CG content 25,678/16.20/42.9 25,678/16.20/42.9 25,717/16.21/42.9 25,707/16.16/42.9 25,678/16.13/42.9
GC content (%) 36.4 36.3 36.4 36.2 36.2
Number of genes 128 133 129 130 130
Protein-coding genes 83 88 84 85 85

bp, base pairs.

Gene map of the chloroplast genome of Morus atropurpurea. Genes shown on the side of the larger circle are transcribed clockwise. Inverted repeats (IRa and IRb) separate the genome into small and large single-copy regions.

The GC content of the M. multicaulis chloroplast genome was found to be 36.2%, which is lower in LSC (33.9%) and SSC (29.3%) regions and higher in IR regions (42.9%). No changes were found to occur in the IR region of the five mulberry species. cpDNA contains 130 functional genes including eight rRNA genes, 37 tRNA genes, and 85 PCGs. Pseudogenes and ORFs were all non-coding.

Eighteen genes, including seven tRNA, seven PCGs, and all rRNA genes were duplicated in the IR regions. M. multicaulis contained 22 genes (eight tRNA, 12 PCGs, and two pseudogenes), with one intron, which is consistent with that found for M. atropurpurea, with the exception of two genes (ycf3 and clpP) that contain two introns (Table 2). Of the 22 genes, 10 are situated within the IR region (4 PCGs, 4 tRNAs, and 2 pseudogenes), 1 in the SSC region (ndhA), and 11 in the LSC region (7 PCGs and 4 tRNAs), and this study also found trnK-UUU has the largest intron that contains the protein-coding gene matK, which is similar to that found in green plants (Zhang et al., 2013).

Genes present in the chloroplast genome of Morus atropurpurea and M. multicaulis.

Function Gene group Gene name
Self-replication Ribosomal RNA genes rrn4.5 (x2) rrn5 (x2) rrn16 (x2) rrn23 (x2)
Transfer RNA genes trnA-UGC (x2)trnF-GAAtrnH-GUGtrnL-CAA (x2)trnN-GUU (x2)trnR-UCUtrnT-GGUtrnW-CCA trnC-GCAtrnfM-CAUtrnI-CAU (x2)trnL-UAAtrnP-UGGtrnS-GCUtrnT-UGUtrnY-GUA trnD-GUCtrnG-GCCtrnI-GAU (x2)trnL-UAGtrnQ-UUGtrnS-GGAtrnV-GAC (x2) trnE-UUCtrnG-UCCtrnK-UUUtrnM-CAUtrnR-ACG (x2)trnS-UGAtrnV-UAC
Small subunit of ribosome rps2rps8rps15 rps3rps11rps16* rps4rps12 (x2)rps18 rps7 (x2)rps14rps19
Lange subunit of ribosome rpl2* (x2)rpl22rpl36 rpl14rpl23 (x2) rpl16*rpl32 rpl20rpl33
RNA polymerase subunits rpoA rpoB rpoC1 * rpoC2
Photosynthesis NADH dehydrogenase ndhA*ndhEndhI ndhB* (x2)ndhFndhJ ndhCndhGndhK ndhDndhH
Photosystem I psaApsaJ psaB psaC psaI
Photosystem II psbApsbEpsbJpsbN psbBpsbFpsbKpsbT psbCpsbHpsbLpsbZ psbDpsbIpsbM
Cytochrome b/f complex petApetL petB*petN petD* petG
ATP synthase atpAatpH atpBatpI atpE atpF*
Large subunit of rubisco rbcL
Other genes Maturase matK
Protease ClpP*
Envelope membrane protein cemA
Subunit of acetyl-CoA-carboxylase accD
C-type cytochrome synthesis ccsA
Component of TIC complex yf1 (x2)
Unknown function Hypothetical chloroplast reading frames ycf2 (x2)ycf68* (x2) ycf3* ycf4 ycf15 (x2)
ORFs orf42 (x2)

Asterisks indicate genes containing one or more introns.

Codon usage

All 85 protein-coding genes in the cpDNA of M. multicaulis and M. atropurpurea were encoded by 53,051 and 53,037 codons, respectively (Table 3). Codon usage strongly reflects the AT tendency. For M. atropurpurea, 63.5% of codons end in A or T, with 73.5% stop codons ending in A or T. Leucine accounts for the highest codon usage (5624), followed by serine (4778), isoleucine (4731), and phenylalanine (3754). These four amino acids represent one third of the total codons. TAA is the most frequent stop codon found, accounting for 1268 uses, which is higher than that of TGA (1079) and nearly twice that of TAG (847) (Table 3). ATG (919) was the most common start codon, with the exception of GTG for rps19 and ACT for rps2.

Codon usage in Morus multicaulis and M. atropurpurea.

Codon Amino acid M. atropurpurea M. multicaulis Codon Amino acid M. atropurpurea M. multicaulis
GGG Gly (G) 542 494 TGG Trp (W) 648 684
GGA Gly (G) 740 759 TGA stop 1079 1032
GGT Gly (G) 551 599 TGT Cys (C) 711 725
GGC Gly (G) 332 350 TGC Cys (C) 436 435
GAG Glu (E) 599 550 TAG stop 847 786
GAA Glu (E) 1245 1368 TAA stop 1268 1306
GAT Asp (D) 1022 1064 TAT Try (Y) 1524 1624
GAC Asp (D) 412 425 TAC Try (Y) 730 690
GTG Val (V) 448 418 TTG Leu (L) 1083 1073
GTA Val (V) 748 728 TTA Leu (L) 1359 1250
GTT Val (V) 797 792 TTT Phe (F) 2369 2343
GTC Val (V) 432 430 TTC Phe (F) 1385 1471
GCG Ala (A) 228 249 TCG Ser (S) 586 578
GCA Ala (A) 431 430 TCA Ser (S) 1017 979
GCT Ala (A) 463 511 TCT Ser (S) 1193 1273
GCC Ala (A) 328 321 TCC Ser (S) 858 864
AGG Arg (R) 632 596 CGG Arg (R) 366 350
AGA Arg (R) 1036 1044 CGA Arg (R) 564 596
AGT Ser (S) 654 718 CGT Arg (R) 383 363
AGC Ser (S) 470 478 CGC Arg (R) 244 236
AAG Lys (K) 1050 1039 CAG Gln (Q) 462 440
AAA Lys (K) 2206 2280 CAA Gln (Q) 1067 1013
AAT Asn (N) 1924 1883 CAT His (H) 907 945
AAC Asn (N) 802 728 CAC His (H) 391 362
ATG Met (M) 919 855 CTG Leu (L) 505 489
ATA Ile (I) 1700 1729 CTA Leu (L) 859 799
ATT Ile (I) 1945 1965 CTT Leu (L) 1172 1065
ATC Ile (I) 1086 1083 CTC Leu (L) 646 581
ACG Thr (T) 332 399 CCG Pro (P) 378 400
ACA Thr (T) 733 689 CCA Pro (P) 723 738
ACT Thr (T) 683 690 CCT Pro (P) 616 730
ACC Thr (T) 593 587 CCC Pro (P) 578 580

Comparison with other Rosales chloroplast genomes

The cp genomes of M. multicaulis and M. atropurpurea contained 83 and 81 SSRs, respectively, of at least 10 bp in size (Table 4). A total of 60, 8, 3, 10, and 2 mono-, di-, tri-, tetra-, and pentanucleotide repeats were found in the M. atropurpurea chloroplast genome. All mononucleotides and 17 other SSRs were comprised of T and A nucleotides, with a high AT content (92.2%). Of the 83 SSRs, 23 were located within gene-coding regions and 60 were located within intergenic spacers. SSRs were rarer in protein-coding genes than in non-coding regions (Rajendrakumar et al., 2007). Fifty-two loci were identical between M. atropurpurea and M. multicaulis, 31 were unique, and three were not found (Table 4).

Distribution of SSR loci in the Morus atropurpurea (M.A) and M. multicaulis (M.M) chloroplast genomes.

Repeat unit Length(bp) Number of SSRs Position in the chloroplast genome (gene name)
M.A M.M M.A M.M
A 10 8 10 3997 (trnK-UUU); 5100; 5998 (rps16); 29085; 49740; 68673; 68688; 114237 (ndhF) 2142 (trnK-UUU); 3980 (trnK-UUU); 5079; 5977 (rps16); 29067; 49740; 68616; 68631; 114154 (ndhF); 116262
11 4 3 53953; 62875; 87528; 116346 9589; 62837; 87467
12 2 3 13603 (atpF); 84635 4830; 53982; 85376 (rpl16)
13 1 13596 (atpF)
14 1 1 128093 128163
15 2 1 9583; 74234 (clpP) 74160 (clpP)
16 1 1 9002 8990
17 1 4846
T 10 18 20 5279; 9801; 24375 (rpoC1); 30690; 30956; 54024; 54921;5 7117 (atpB); 58017 (rbC1); 62648; 66988; 68800; 70966; 74032; 116849; 122289; 130417 (ycf1); 132174 (ycf1) 66; 5258; 8582; 9802; 68743; 70892; 73958 (clpP); 83130; 14098; 14919; 24357 (rpoC1); 30672; 30938; 54024; 57098 (atpB); 62610; 66927; 116784; 130487 (ycf1); 132244 (ycf1)
11 7 6 127; 526; 8593; 59604; 74750; 78755 (petB); 131276 (ycf1) 513, 34264; 69552; 78684 (petB); 122351; 131346 (ycf1)
12 9 5 12711; 27635 (rpoB); 34289; 37832; 57588; 68549; 69620; 72545; 85868 (rpl16) 27617 (rpoB); 57549; 59565; 72471; 85809 (rpl16)
13 3 5 9225; 13293 (atpF); 128515 12703; 13286 (atpF); 68491; 81352; 128585
14 1 5 63903 9213; 51829; 63865; 74676 (clpP); 86927
16 1 81423
17 2 1 49438; 84685 49475
19 1 116631
AT 10 2 1 68872; 115739 (ndhF) 11566 (ndhF)
12 1 3 10814 5522; 118643; 118871
TA 12 4 1 5543; 21253 (rpoC2); 118731; 118839 21234 (rpoC2)
TC 10 1 1 64630 (cemA) 645927(cemA)
TAT 12 1 49786
TTC 12 1 1 70983 70909
AAT 12 1 1 128481 128565
ATTT 12 1 1 14192 62140
16 1 14187
AAAT 12 2 2 24069 (rpoC1); 46696 (ycf3) 24056 (rpoC1); 46731 (ycf3)
TATT 12 1 1 24406 (rpoC1) 24388 (rpoC1)
ATTA 12 2 2 34075; 116528 33980; 116443
TTTA 12 1 62179
TCTT 12 1 1 111648 111575
TTAT 12 1 117879
AAAG 12 1 1 135264 135331
AAGGA 15 1 1 14037 (atpF) 14021 (atpF)
ATTTC 15 1 24509 (rpoC1)

The borders of the two inverted repeats (IRa and IRb) with the LSC and SSC regions play an important role in the expansion and contraction of the chloroplast genome (Goulding et al., 1996). It is believed that the locations of the SSC/IR and LSC/IR junctions are markers of chloroplast genome evolution (Zhang et al., 2013). The IR junction among the potential impact of these changes in the cp genome of Morus was compared.

Four complete chloroplast genome sequences of Rosales and the sister group Cucurbitales were selected, namely, M. atropurpurea, M. multicaulis, M. mongolica, M. notabilis, Rosa odorata var. gigantea, Cucumis melo subsp melo, and Corynocarpus laevigatus. The IR boundaries of cpDNA from M. atropurpurea and M. multicaulis were very similar (Figure 3). The IRb-SSC junction was found to be located at the ndhF gene, and the ndhF and ycf1 genes overlapped by 32 bp in C. melo subsp melo. The IRa-SSC was located in ycf1, resulting in the formation of a ycf1 pseudogene. The boundary of the LSC/IR was located within the rps19 gene, also resulting in the formation of an rps19 pseudogene, which is consistent with the findings of a previous study (Nazareno et al., 2015).

Comparison of the junction between IR and SC regions among Rosales and its sister group. MA: Morus atropurpurea; MM: M. multicaulis; MG: M. mongolica; MN: M. notabilis; CM: Cucumis melo subsp melo; CL: Corynocarpus laevigatus; RO: Rosa odorata var. gigantea.

mVISTA (Frazer et al., 2004) was used to compare sequence identity between the six cpDNAs, referring to the annotation of the N. tabacum cpDNA (Figure 4). Although some divergent regions were found, Rosales cpDNAs were found to be rather conserved through the complete aligned than their non-coding regions. For M. atropurpurea, M. mongolica was the closer relative, followed by M. multicaulis, M. notabilis, P. pyrifolia, Prunus kansuensis, F. vesca subsp vesca, and N. tabacum.

Y-scale represents identity, ranging from 50 to 100%. Genomes are arranged according to the number of conserved bases relative to Rosales.

The complete chloroplast genomes of Rosales clades were used to construct the phylogenetic tree in MEGA6.0 via the ML (Figure 5) and NJ methods (Figure 6). The two methods group the Morus species together. The ML and NJ methods grouped M. atropurpurea and M. mongolica together. However, we cannot conclude that M. atropurpurea and M. mongolica have a close genetic relationship, because the cp genomes of other Morus species were not sequenced. Moreover, further research into Morus species is needed in order to reach a conclusion.

Phylogenetic analysis of Morus species using the complete chloroplast genome by the ML method.

Phylogenetic analysis of Morus species using the complete chloroplast genome by the NJ method. Nicotiana tabacum is included as the outgroup to root the tree.

DISCUSSION

In recent years, researchers have used cpDNA for the study of plant evolution, along with the published chloroplast data available in the NCBI database (Drew et al., 2014). In our study, we prudentially selected cpDNAs of different taxa from the NCBI database that were potentially published. Additionally, long-branch attraction will mislead to a wrong phylogenetic tree. Research has shown that M. mongolica and M. indica are wild species of Morus L. (Yang and Yoder, 1999), whereas, M. atropurpurea and M. multicaulis are cultivated species of Morus L. In the present study, the complete chloroplast genome sequence of M. atropurpurea was determined and compared with that of M. multicaulis and the wild species of Morus. The genome sequence, the size of the LSC, IR, SSC, and the CG content, among other variables, were analyzed providing detailed information for phylogenetic studies of the chloroplast. The results revealed that the size of the M. atropurpurea chloroplast genome is 159,113 bp, which is 41 bp shorter than that of M. multicaulis and 654 bp longer than that of M. mongolica. Moreover, there were few differences in the length of the IR and SSC regions of the cpDNA from all five species, with differences accounted for by the LSC region. Analysis of the results also indicated that these species are closely related and this will be confirmed by construction of the phylogenetic tree.

The expansion and contraction of IR are common evolutionary events in plants (Liu et al., 2013). It is believed that the locations of SSC/IR and LSC/IR junctions are markers of chloroplast genome evolution (Zhang et al., 2013). Here, we compared the positions of the IR/SC boundary in six complete cpDNA sequences. The IR boundaries of Morus species followed the same pattern in terms of the order of genes and structure, except for the IRb/SSC and IRa/LSC boundaries. In the IRb/SSC junction, 52 bp of the ndhF gene was located in IRb, with the rest located in the SSC in M. atropurpurea; this differed in Morus species. In the IRa/LSC boundary, the trnH-GUG gene was 175-bp away from the boundary of IRa/LSC in M. atropurpurea, 242-bp away in M. multicaulis, and 23-bp away in M. mongolica. The IR boundary showed that M. atropurpurea and M. multicaulis are closely related, and have a closer genetic relationship to M. mongolica than to M. notabilis. Studies based on IR/SC junction regions and other variable regions from different Morus species would be of great help in systematics. In addition, the information generated from such studies would be useful for taxonomic analyses of other species of Morus, other genera within Moraceae, and other families within the same subclass. The cpDNA sequence of cultivated Morus described in the present study will contribute to further studies on molecular breeding, phylogenetics, and genetic engineering.

Most cpDNAs are AT rich (AT content above 60%), have conserved regions with lower AT contents, and have unevenly distributed AT contents (Cai et al., 2006). cpDNA from M. atropurpurea and M. multicaulis exhibited the same features, and the AT content in the whole cpDNA, SSC, LSC and IR regions was 63.8, 70.7, 66.1, and 57.1%, respectively, with no changes observed between the two mulberry species (Table 1). Similarly, regions with a high AT content harbor more variation, such as hypervariable regions and SSRs. SSR polymorphisms between M. multicaulis and M. atropurpurea all involved A or T mutations. These phenomena indicate that a positive correlation exists between sequence divergence and AT content, and that there is a bias toward A and T changes over G and C changes in plant cpDNAs.

The rpl21 gene is only present in the plastomes of ferns and bryophytes (Steane, 2005) and the infA gene is known to have been transferred to the nucleus and lost from almost all known rosid plastomes (Millen et al., 2001). The Morus plastome also contains two pseudogenes, ycf15 and ycf68. ycf15 is not believed to be a protein-coding gene (Schmitz-Linneweber et al., 2002). The ycf15 gene fragment indicates that it is a remnant of an ancestral functional gene. The deletion observed in the ycf68 gene, which causes the frame-shift, does not appear to have been a sequencing artifact, as the coverage and read quality in the concerned region were high.

The SSRs identified in M. atropurpurea, serving as important molecular markers, can be applied to further population genetics studies (Katti et al., 2001; Shaw et al., 2007). We identified 83 and 81 SSRs in the M. multicaulis and M. atropurpurea cp genomes, respectively. Due to their variability at inter- and intrapopulation levels, these SSRs may be useful in evolutionary studies. Future research should focus on the validity of SSRs in phylogenetic and ecological studies of Morus. Data on the SSRs of Morus are available and were used in the present study. We found that the numbers of SSRs in the complete cpDNAs of different Morus species were almost identical. A number of SSRs were located within the same gene (Nguyen et al., 2015). For example, dinucleotides were observed in rpoC2, cemA, and ndhF, and trinucleotides were observed in the non-coding region. Moreover, three mononucleotides were observed in the ycf1 gene and two mono-, two tetra-, and one pentanucleotide SSRs were found in the rpoC1 gene. SSRs distributed in coding genes between M. atropurpurea and M. multicaulis were similar, containing atpF, ycf1, cemA, atpB, rpoC2, and ndhF, which was consistent with the findings of Kong and Yang (2016).

The nucleotide sequence and structure of the complete chloroplast genomes of M. multicaulis and M. atropurpurea, and the sequence differences between Morus species and other species presented in this study will contribute to future evolution and ecological studies.

The cpDNA sequences of Morus species, including M. mongolica, M. indica, and M. notabilis, have been reported; however, data on the cpDNA of cultivated Morus species are limited. The complete cpDNA sequences of M. atropurpurea and M. multicaulis reported here enhance genome information on Morus and contribute to the study of germplasm diversity. These data represent a valuable source of markers for future studies on Morus populations. Moreover, the complete cp genome sequence also provides data on functional protein variability within the chloroplast.

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